An acquisition of Zenbe's email product and the hiring of three of its engineers explains the "it's not email" stance of Facebook's announcement of messaging yesterday.

The most quoted sound bite from yesterday’s messaging announcement, “it’s not email,” now has an explanation behind it: Facebook has hired engineers from Zenbe, which has announced on its own website that the two have an arrangement. Zenbe’s home page congratulates Facebook on the launch of the new messaging product, and links to a blog post explaining the details of the two companies’ interactions.

We here at Zenbe are happy to have played a small role in this: a few months ago, three of our engineers joined Facebook as part of a talent acquisition. We believe that messaging and collaboration on the web are on the cusp of a new wave of innovation, and we’re excited to be playing a role in that.

Zenbe reportedly shut down its own, self-branded email product on October 8, and it turns out the startup actually sold its intellecutal property to Facebook. Three of Zenbe’s engineers joined the social network in Palo Alto. The other engineers still working at Zenbe remain in New York, where the company continues to develop a product called Shindig. It promises to tap Facebook’s social graph for location-based photo sharing with real time updates.

Zenbe already has two other products fully live with some features in common:

Lists is a simplified list-sharing tool optimized for the iPhone and iPad but that friends using other platforms can also use.

Shareflow combines chat, email and workflow, without requiring system log-ons.

The Zenbe arrangement only partly resembles a recent pattern of buying startups and hiring their founders. Facebook acquired the file-sharing startup Drop.io last month, and in August picked up FriendFeed.

These acquisitions together help explain how Facebook could incorporate so many different types of communications services into one messaging service, or “social inbox.” But I’m wondering about one of the features announced yesterday that hasn’t yet manifested itself in an acquisition and would seem like something that might need acquiring: texting a la short messaging service, or SMS.

Readers, do you suspect that there are additional Facebook acquisitions related to the new messaging service? What companies in this space might be in play — or does the social network already have a full plate?